1. Field of the Invention
The present invention relates to a method for the treatment of process waste resulting from the synthesis of thiophene compounds and their derivatives, which compounds are used in the manufacture of conducting polymers, said method providing phosphorus pentasulfide and a treatment residue suited for further utilization.
2. Description of the Prior Art
Sulfides of phosphorus are conventionally produced by heating elemental sulfur and phosphorus above their melting points and then cooling the process product. Phosphorus pentasulfide, referring to the compound having the formula P.sub.4 S.sub.10, can be alternatively produced in a similar manner from sulfur and red phosphorus. Pure phosphorus pentasulfide is obtained by either using pure raw materials combined in an accurately determined sulfur-to-phosphorus ratio of the reactants, or, if the reactant materials are impure, through separate distillation of the reaction product from the heating of the sulfur and phosphorus. The reaction conditions of a manufacturing process for phosphorus sulfides can be determined from equilibrium curves. For further details refer to, e.g., such textbooks as Ullmann, Vol. 18, p. 369 and Kirk-Othmer, Vol. 17, p. 503.
Thiophenes or their derivatives can be employed in the manufacture of electrically conducting polymers according to patents granted to Neste Oy, refer to, e.g., U.S. Pat. Nos. 5,151,221 and 5,219,492 and WO 92/10352. To produce thiophene or its derivatives, a process has been proposed based on treating succinic anhydride or substituted succinic anhydride with a compound, containing sulfur and phosphorus. Such a manufacturing process gives large amounts of process waste, chiefly containing phosphorus, sulfur, potassium and oxygen, and further, minor amounts of carbon, hydrogen and sodium.
It is an object of the present invention to achieve a method for treatment of the above-described process waste that converts the waste into products suitable for further utilization. Because the process waste contains phosphorus and sulfur and because phosphorus pentasulfide is required in the manufacture of thiophene compounds, a person skilled in the art finds it logical to aim at producing particularly phosphorus pentasulfide from said waste.